Our aim is to understand the molecular organization and the control of genetic expression of the mouse X and 7 chromosomes. We have isolated and characterized DNA fragments specific to these two mouse chromosomes. We are using these chromosome specific probes to study the mechanism of X inactivation and its spreading in an X;7 chromosomal translacation. We plan to characterize one of the X-specific fragments which is expressed in mouse liver, by nucleotide sequencing of the genomic clone and comparison with its corresponding cDNA. DNA methylation of X-chromosome specific fragments will be compared between male and female mouse DNA to determine if methylation changes occur with X chromosome inactivation. Chromatin structure changes associated with X inactivation will be studied by determining the nuclease sensitivity of specific X-chromosomal loci during inactivation. A restriction enzyme will be chosen for these experiments that distinguishes maternal and paternal X chromosomes on the basis of a DNA restriction site polymorphism (between mouse strains or mouse species). For these experiments, chromatin will be extracted from cloned cell lines that carry either the paternal or maternal X chromosome in an active state, or from extraembryonic membranes which demonstrate preferential paternal X inactivation. To construct a complete correlated genetic and molecular mouse X-chromosome linkage map, mouse X-chromosome fragments already isolated and additional fragments will be localized to the mouse X chromosome by in situ hybridization and the use of recombinant inbred strains of mice. To construct a molecular map of the developmentally important albino locus, a mouse chromosome 7-fragment that we have isolated will be used to isolate additional DNA fragments around that locus. The fragment isolated is located in the overlapping portion of two albino deletions, c3H and c6H. These deletions affect embryogenesis and modify the expresion of several liver enzymes. Chromosome 7-specific DNA fragments will also be used to study the effects of X inactivation on the autosomal portion of the X;7 chromosomal translocation.